Antenna diversity can significantly improve the performance of a wireless receiver system but also increases the cost. In order to keep the cost down, most wireless receiver systems in consumer applications use a single antenna receiver. In many handset units, the components that cause cellular transmissions (such as CDMA) are located in relatively close proximity of other wireless receivers (such as WiFi or mobile TV receivers) disposed in the handset unit. FIG. 1 shows a number of processing units of a conventional handset 100. Handset 100 is shown as having a CDMA cellular unit 110, a Bluetooth transceiver 120, a WiFi or WiMax transceiver 130, and a mobile TV receiver 140. Each of these four units is shown as having a single dedicated antenna.
FIG. 2 shows a handset unit 200 that includes, in part, a cellular CDMA transceiver 210 having an antenna 215, and a mobile TV diversity receiver 220 having antennas 225 and 230. Because of their close proximity, transmissions from cellular transceiver 210 often couple significantly to receiver 220. This problem is further compounded by the relatively high power that transceiver 210 requires to transmit data.
Referring to FIG. 1, in order to minimize the impact of the strong transmissions by CDMA cellular unit 110, on the receivers 120, 130, and 140, conventional handset units typically include a filter (that has a relatively sharp filtering characteristics) at the input of the receivers (e.g., WiFi or Mobile TV receivers.) This filtering operation is typically performed using a SAW filter which is costly and consumes a relatively large space.
FIG. 3A shows a conventional diversity receiver system 300 having two receiving channels, namely a first receiving channel 320 and a second receiving channel 340. First receiving channel 320 is shown as including a low-noise amplifier 322, a mixer 324, a filter 326, and an amplifier 328. Likewise, second receiving channel 340 is shown as including a low-noise amplifier 342, a mixer 344, a filter 346, and an amplifier 348. Diversity baseband processor 310 combines the signals it receives from first and second receiving channels 320 and 340, using any one of a number of well-known algorithms, to generate and output the sensed signal.
FIG. 3B shows a conventional diversity receiver system 350 having a pair of receiving channels 320 and 340 that are similar to the receiving channels 324 320 of FIG. 3A. Diversity receiver system 350 includes a first diversity baseband processor 325 that receives the output signal of amplifier 328, and a second diversity baseband processor 345 that receives the output signal of amplifier 348. Diversity baseband processors 325 and 345 communicate with one another to asses, in conformity with a selected diversity algorithm, the signal quality or signal-to-noise ratio (SNR) from antennas 306, 308, and to weight the signal received from each channel based on this assessment. For example, poor SNR from antenna 308 compared with the SNR from antenna 306 causes the baseband processors to assign less weight to the signal from antenna 308 than the signal from Antenna 306.
FIG. 4 shows an example of the EM radiation spectrum seen by a handset unit such as handset unit 200 shown in FIG. 2. Signal 420 is transmitted by transceiver 210 and is commonly referred to as the blocker signal. Signal 430, which has a frequency that is close to the frequency of signal 420, is the desired TV channel being received by receiver 220. Signal 420 is often a strong signal and thus can severely interfere with the reception of signal 430.
To attenuate the blocker signals, a SAW filter is commonly used in the receiving channel. FIG. 5 shows a typical transfer function 510 of a SAW filter, which as is seen, has a sharp frequency roll-off to attenuate the blocker signal 420. SAW filters are commonly used in front of sensitive receivers to provide this effect. One disadvantage of SAW filters is that they are costly and introduce non-negligible loss, thereby directly degrading the noise figure of the receiver.
Conventional diversity receivers use a SAW filter in each of the diversity channels. FIG. 6 shows a handset unit 600 that includes diversity receiving channels 610, 630, diversity baseband processor 650, and a transceiver 660. Receiving channel 610 includes a SAW filter 612, a low-noise amplifier (LNA) 614, a mixer 616, a filter 618, and an amplifier 620. Receiving channel 640 includes a SAW filter 632, an LNA 634, a mixer 636, a filter 638, and an amplifier 640. Baseband processor 650 receives the output signals of amplifiers 620 and 640, and combines them in accordance with any one of a number of well-known algorithms to generate and output the sensed signal that is received. As was described above, the inclusion of SAW filter 632 in the second receiving channel 630 increases the cost of handheld unit 600 and is thus undesirable.